I am new in Buddhism since the past 6 months and been reading the Pali Canon(English Translations). I have been reading about Karma and especially about how Hatred/Aversion/Ill-will leads to Realm of Hell in the next birth.

Now I have a situation in my life I would like to discuss. In my life over a period of about 2 to 2.5 years I had developed Aversion towards one of my family members (actually my sister-in-law - My brother's wife). Now since we were staying in the same house my aversion was present whenever she was around. This is there for a period of 2.5 years like I said until I recently turned towards the Buddha's teachings. His teachings changed my outlook and I developed a mind of non-hatred at first and then a mind of Loving-Kindness towards that person. Now I would like to put an effort to keep this state of Loving-Kindness for the rest of my life.

My understanding of Kamma from the Buddha's teachings says that Unwholesome Kamma and Hatred especially leads to realm of Hell (albeit temporary). I would like to know whether having this unwholesome Kamma for a long period (say 2.5 years) mean that my next birth would be definitely in the Hell realm?. Can it be possible in my life to be good and avoid an unfortunate rebirth to hell or is it impossible now considering the long period I had carried unwholesome root?.

---The trouble is that you think you have time---
---Worry is the Interest, paid in advance, on a debt you may never owe---
---It's not what happens to you in life that is important ~ it's what you do with it ---

If you believe in true rebirth it has to do with your deeds more so than your thoughts. So your bad karma would build but you can lessen its effect by generating good karma in your life now that you have better insight. Not just on this matter with your sister-in-law but with all kind deeds you can do and experiences with everyone. The best analogy I have heard on this matter is relating bad karma to salt and good karma to water, if you have a cup of salt and a cup of water you aren't going to want to drink it. But if you have a cup of salt and a lake of water you wouldn't even be able to tell the difference.

I highly doubt your previous actions would warrant being reborn in the realm of Hell especially with all of the good will you have been spreading since then.

Good luck to you and your practice, you should worry less about the future.

I am new in Buddhism since the past 6 months and been reading the Pali Canon(English Translations). I have been reading about Karma and especially about how Hatred/Aversion/Ill-will leads to Realm of Hell in the next birth.

Now I have a situation in my life I would like to discuss. In my life over a period of about 2 to 2.5 years I had developed Aversion towards one of my family members (actually my sister-in-law - My brother's wife). Now since we were staying in the same house my aversion was present whenever she was around. This is there for a period of 2.5 years like I said until I recently turned towards the Buddha's teachings. His teachings changed my outlook and I developed a mind of non-hatred at first and then a mind of Loving-Kindness towards that person. Now I would like to put an effort to keep this state of Loving-Kindness for the rest of my life.

My understanding of Kamma from the Buddha's teachings says that Unwholesome Kamma and Hatred especially leads to realm of Hell (albeit temporary). I would like to know whether having this unwholesome Kamma for a long period (say 2.5 years) mean that my next birth would be definitely in the Hell realm?. Can it be possible in my life to be good and avoid an unfortunate rebirth to hell or is it impossible now considering the long period I had carried unwholesome root?.

Just regretting your actions through Buddha Teachings is a huge success since its very hard to lose your Aversion so easily. And if you start realizing the true meaning behind Buddhism then I am sure your karma will not lead you to hell realm.
So just focus on buddha's teachings from the bottom of your heart and devote yourself and I am sure you will fine.